Image Contrast Flashcards
define image contrast
the degree of difference between the light and dark areas of a radiograph
What makes the blacks, whites, and grays on a radiograph?
differential absorption
subject contrast
component of image contrast determined by size, shape, and x-ray attenuating characteristics of the subject
define differential absorption
the difference between photons absorbed and those that penetrate the body
different body structures absorb x-ray photons to ____________ extents
different
bones are denser and ________ more x-ray photons than those filled with air such as lungs
absorb
attenuation
removal of any photons from the primary beam before striking the IR
attenuation is the reduction of:
the intensity of an x-ray beam
causes of attenuation:
differential absorption
scatter
absorption
x-ray photons that are attenuated by the body and do not reach the IR
transmission
x-ray photons that pass through the body and reach the IR
What terms describe the beam entering the patient?
primary
useful
entrance
primary beam
x-ray beam as it is upon exiting collimator and exposing patient
What terms describe the beam leaving the patient?
remnant
exit
remnant beam
x-ray beam that remains after interaction with patient when exiting the patient to the IR
the remnant beam is composed of what?
transmitted and scattered photons
radiopaque
body structures that absorb x-ray photons
(bones)
radiolucent
less dense body structures that have a lower probability of absorption
(air filled lungs)
photons absorbed show up as _______ on a radiograph
white
photons that pass through body and reach IR show up as _______ on a radiograph
black
technical term for absorption within the patient
Photoelectric Effect
denser tissue _________ the probability of photons being absorbed
increases
structures filled with air __________ the probability of absorption
decreases
what causes the shades of gray?
photons that aren’t completely absorbed or transmitted
high contrast:
large differences in brightness
high contrast images have mostly:
areas of black and white
only few shades of gray
a high contrast image is also referred as:
short-gray scale
short gray scale contrast
mostly black and white, few shades of gray
low contrast:
small differences in brightness
low contrast images have mostly:
areas of gray
only few areas of black and white
a low contrast image is also referred as:
long gray-scale
long gray scale contrast
many shades of gray
very little black and white
the total brightness levels that an imaging monitor is capable of displaying is referred to as:
bit depth
high contrast = ______ gray-scale
short
low contrast = ______ gray-scale
long
8 factors that affect image contrast
kVp
differential absorption
computer processing and display
filtration
Compton scatter
grids
OID
beam restriction
high kVp =
low contrast
long gray-scale
why does increasing kVp decrease contrast?
it creates a more penetrating beam = less differential absorption = longer gray-scale = low contrast image.
low kVp =
high contrast
short gray-scale
Why does decreasing kVp increase contrast?
it creates a less penetrating beam, increasing differential absorption, which creates a shorter gray-scale, thus high contrast
higher levels of differential absorption =
higher levels of contrast
Look Up Table (LUT)
takes place after the exposure is made and image is processed by imaging system software
added filtration will result in:
decrease in differential absorption
longer gray-scale thus lower contrast
filtration and image contrast has what kind of relationship?
indirect
higher filtration =
harder average beam
lower contrast/long gray-scale
lower filtration =
softer average beam
higher contrast/short gray-scale
Compton scatter
x-ray photon that interacts with the body and “scatters” off its original trajectory
Compton scatter contributes no useful:
information to the image
Compton scatter causes a uniform exposure on the IR, causing:
a loss of visible image contrast
radiographic grid
device used to reduce the intensity of scatter radiation in the remnant beam
grid clean up
ability of a grid to absorb scatter radiation
a grid _______________ the scattered photon before it can strike the IR and reduce image contrast
catches or attenuates
OID and image contrast share a _________ relationship
direct
increased OID =
increase image contrast
the most common type of beam restriction is:
use of collimation
increasing collimation:
reduces scatter, leading to a higher image contrast
Raw numbers (bar graph) of what is occurring in the body (absorption/transmission) will determine?
dose
Percentages of what is occurring in the body (absorption/transmission) will determine?
contrast
effects of kVp on the x-ray beam:
increasing kVp creates a more penetrating beam
decreasing kVp creates a more less one
effects of kVp on patient dose and on absorption
increasing kVp causes a greater dose due to greater % of beam passing through patient, which has a decrease on absorption
vice versa
why does increasing kVp increase dose?
increasing kVp creates a bigger beam
Why does increasing kVp create a bigger beam?
more electrons are called over which creates more photons
increasing kVp does what to dose and absorption?
increases dose, but decreases the percentage of absorption
will an increase in kVP cause an increase or decrease to the photoelectric?
decrease
describe the relationship between filtration and image contrast
indirect (greater the filtration the more low photons are removed)
a photon that does not strike the IR, what type of photon is this?
attenuating photon
in the digital age, what is the controlling factor for image contrast?
look up table (LUT)
a decrease in kVp will cause the average amount of transmission to increase or decrease?
decrease
why does beam restriction improve image contrast?
reduces amount of scatter (less matter = less scatter
what are the two different ways an x-ray photon can become attenuated?
scatter and absorption
what produces subject contrast?
differential absorption
using one word, describe why increasing OID improves image contrast
divergence
the total number of grays a system is capable of displaying in a radiograph image is referred to as what?
bit depth
decreasing kVp will have what effect on differential absorption?
increases
what is the controlling factor for subject contrast?
kVp
illustrate how an increase in kVp can cause an increase in dose and a reduction of contrast at the same time:
2 kvp lines with one slightly higher and fatter toward when it gets to the bottom
dose is in middle section and contrast is in wider section near bottom
